1. Field of the Invention
The present invention relates to a data transfer control method and data transfer circuit capable of completing high-speed data transfer by eliminating unnecessary data when transferring image data, for a personal computer or word processor, while shifting it.
For example, when transferring image data which is generated from outline fonts to a video RAM, it is necessary to shift and transfer the data, and it is desirable to achieve high-speed processing of such transfer.
2. Description of the Related Art
In the past, when transferring image data while shifting the data, the size of the data in units of boundary was held as data, and if N data were received for each horizontal line of image data, N+1 writing operations were automatically performed, regardless of whether or not the image data was meaningful.
As a result, there arose a state in which part of the data transferred was unnecessary data.
The prior art includes an "apparatus for reading data from a bit-map display memory," as disclosed in the Unexamined Patent Application Publication (JP-A) Sho 63-278181. However, while this "apparatus for reading data from a bit-map display memory" was capable of continuously reducing the burden on the CPU by setting the amount of shift into a shifting means, there is no mention of useless data which is generated by the shifting of the data edges.
As will be explained, later and in detail, with reference to the accompanying drawings, when image data, starting from a boundary partition, which boundary is the unit of data writing, is shifted and transferred, the right end data of a horizontal line can contain a part that is meaningless as image data.
If the read range of image data that was input (read) at, for example, four times per horizontal line is shifted, after which it is output (written) at five times per horizontal line, the last write data becomes unnecessary. That is, the image data written at the fifth write operation is absolutely no data which is valid meaningful image data. This is because there is no determination made during the read operation as to how far the meaningful image data extends and the image data is defined in coarse boundary units.
After shifting, if the starting position of the image data does not coincide with a boundary partition, depending upon the size of the image data, and also on the relative positions between the boundary unit, which is the reading unit, and the image data, it is possible to write data which was read in with N read operations for each line by either N or N-1 write operations. However, in the prior art, because N+1 write operations were normally performed, either 1 or 2 write operations were unnecessary.